The present invention relates to an electrophotographic process in which a composite photosensitive member is used.
A composite photosensitive member for use in the electrophotographic process is generally formed by applying on a conductive carrier a photoconductive semiconductor having a charge generating function by light image projection and a transparent photo semiconductor having charge retentivetransport function in order. Japanese Patent Application Publication Nos. 16,198/68 and 5,349/70 and U.S. Pat. No. 3,725,058 disclose a composite photosensitive member in which a pure Se or a Se having doped Te or As for improving light sensitivity, particularly, color sensitivity is used as photoconductive semiconductor and polyvinylcarbazoles vinyl polymers are used as transparent photo semiconductor. Japanese Pat. Laid-opened No. 67,660/75 discloses a composite photosensitive member in which CdS, ZnCdS, etc. are used as photoconductive semiconductor. Japanese patent application Publication No. 14,272/74 discloses a composite photosensitive member in which a film layer such as epoxy resin, polyurethane resin, silicone resin is interposed between a conductive carrier member and a photoconductive semiconductor in order to emphasize a combining force of these members. Japanese patent application Publication No. 26,148/74 discloses a composite photosensitive member in which a film layer of high resistivity semiconductor consisting of Se of high purity, As.sub. 2 S.sub.3 is interposed between a conductive carrier member and a photoconductive semiconductor in order to improve charging characteristic and dark decay characteristic.
According to the experiment of the present inventor, however, there is found that following problems arise for the above described composite photosensitive member
(1) If the composite photosensitive member is subjected to charging and exposure processes repeatedly the dark decay becomes large and the amount of residual charges become increased so that charged potential becomes decreased. PA1 (2) If amorphous Se having Te etc. added thereto for increasing sensitivity is used as photoconductive semiconductor the tendency of the above term (1) becomes further increased. PA1 (1) Dark decay is very small, PA1 (2) Residual potential at bright portion of the light image is low. PA1 (3) Exhausted phenomenon by repeating charging and exposure steps, that is, increasing of dark decay, lowering of charged potential and increasing of residual potential should not be large.
The above described composite photosensitive member is applied to an electrophotographic apparatus for carrying out, for example, Carlson process, therefore, a fogging is generated on a duplicated copy in case of repeated utilization and the contrast of the image is largely decreased. If the above composite photosensitive member is applied to an electrophotographic apparatus in which a plurality of duplicated copies are obtained by repeating the development and transfer steps without destroying electrostatic latent image once formed by only one exposure the concentration of the duplicated copy becomes lower as the dark decay is increased, so that even though such low concentration is compensated the excellent result can not be obtained since the compensating condition is not constant.
That is, in the electrophotographic apparatus for forming duplicated copy with the use of the composite photosensitive member following conditions are required.
Means for making dark decay small is as follows.
(i) A method of making the amount of impurities in the photoconductive semiconductor small. For example, in case of using amorphous Se as photoconductive semiconductor it should be used one having high purity of more than 99.999%. However, high purity Se has color sensitivity only at shorter wavelength than that of blue color so that in fact its color sensitivity must be increased by doping and thus doping must be effected to increase sensitivity. Therefore, decrease of dark decay and increase of light sensitivity and color sensitivity are contrary to each other.
(ii) A method of controlling injection of charges into photoconductive semiconductor from conductor with the use of electrical barrier formed between the conductor and the photoconductive semiconductor by interposing organic insulator or inorganic high resistive semiconductor therebetween or barrier potential formed by contacting the photoconductive semiconductor with the conductor of which its material is selected to form barrier potential. In this method if the amount of the barrier potential is selected to a high value the dark decay can be made small. However, if the amount of the barrier potential is made too high resulting in obtaining of high residual potential at bright area in the light image projection the electrostatic contrast becomes lower. In particular, in case of repeating charging and exposure steps charges accumulated in the photoconductive semiconductor are increased and the residual potential is increased to a value not used in practical by repeating charging and exposure steps few times to dozens of times. In addition the dark decay is abrupty increased due to release of charges accumulated in the photoconductive semiconductor.
As described above, decrease of dark decay, increase of light sensitivity and color sensitivity, and decrease of residual potential are contrary to each other, but even if both characteristics are sufficient with each other and a composite photosensitive member having small dark decay and low residual potential is obtained the initial characteristic can not be maintained due to fatigued phenomenon by repeated utilization and thus durability thereof becomes short.